Women who use the Ortho Evra birth-control patch face twice the risk of developing blood clots than those who take the pill, the patch's manufacturer said late Thursday, citing recent company-funded research.

The finding comes from one of two studies comparing the patch and pill, said Ortho Women's Health & Urology, maker of the once-a-week patch. The Raritan, N.J.-based company is owned by Johnson & Johnson.

The first study found no increased risk of clots while its findings on the risk of stroke or heart attack are still being evaluated. Meanwhile, interim results from the second study suggested a twofold increase in the risk of venous thromboembolic events, or clots in the legs and lungs, in women using the patch, Ortho said.

The company said that the risk of clots remains rare and that they have been reported as a potential risk of all hormonal contraceptives. Release of the interim results comes four months after the Food and Drug Administration warned women that the increased levels of hormones released by the patch put them at higher risk of blood clots and other serious side effects. Ortho said it shared the results of the latest studies with the FDA.

Additions to the patch label made in November warned women that they would be exposed to about 60 percent more estrogen than those who use birth-control pills.

Since the patch went on sale in 2002, more than 4 million women have used it.

An investigation by The Associated Press found last year that patch users die and suffer blood clots at a rate three times higher than women taking the pill. About a dozen women died in 2004 from blood clots believed linked to use of the patch, the AP reported, citing federal death and injury reports. Dozens more suffered strokes and other clot-linked problems.

Health officials warn that women who smoke should not use the patch, since smoking increases the risk of stroke and heart attack.